1. Field of the Invention
This invention relates to a recording sheet and a method for producing the sheet. In particular, this invention relates to an improvement in a recording sheet in which a phenol resin is used as a developer.
2. Description of the Prior Art
Recording sheets have been known in the past in which the coupling reaction of an electron donating colorless organic compound (hereinafter referred to as a color former) and an electron accepting solid acid (hereinafter referred to as a developer) is utilized, for example, the pressure-sensitive copying papers as described, e.g., in U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,548,366, 2,712,507, 2,730,456, 2,730,457, and 2,972,547; the heat-sensitive copying papers as described, e.g., in U.S. Pat. No. 2,939,009; and the recording materials as described, e.g., in German OLS No. 1,939,624.
Clays (such as terra alba, activated clay and attapulgite), organic acids (such as succinic acid, tannic acid, gallic acid, salicylic acid and phenols) and acid polymers (such as phenolformaldehyde resins) are known as a developer. In particular, phenol resins are somewhat different from other developers, probably due to their polymeric property, and are particularly advantageous from the standpoint of practical use. Numerous references in the prior art with respect to phenol resins are known, for example, as disclosed in U.S. Pat. Nos. 3,501,331, 3,669,711, 3,427,180, 3,445,721, 3,516,845, 3,634,121, 3,672,935 and 3,732,120.
According to the prior art, a phenol resin is dissolved in an organic solvent and, if necessary, after dispersion into an aqueous solution to prepare an aqueous dispersion, coated on a support. However, conventional developer sheets thus obtained do not have sufficient developability and tend to yellow. In addition, the light resistance of the developed color images formed on these sheets is not sufficient from a practical standpoint.